The invention relates to polyolefinic multilayer film comprising a core layer of polypropylene and at least one outer layer. More specifically, the invention relates to a biaxially oriented polypropylene film that does not distort in the presence of food oils.
Biaxially oriented polypropylene film laminations are commonly used in the snack food packaging industry. In particular, potato chip packaging is a very large volume application.
In the case of potato chip packaging, oils such as cottonseed oil, corn oil, and soybean oil are present on the potato chip surface. Some of this oil on the potato chip surface transfers to the package""s internal surfaces. Similarly, other types of baked food snack products also have oils that are known to swell the polypropylene. Since these food oils are non-polar, the polypropylene film absorbs them quite readily, particularly at elevated temperatures. This absorption of food oils by the film gives a distorted appearance to the entire package by causing dimpling in the film.
This surface distortion effectively destroys the surface gloss of the package and gives the appearance that the package has been mishandled. This distortion is undesirable to snack food manufacturers, as they would like to represent the product as being fresh or new to the shelf.
Oriented polypropylene films currently available on the market have attempted to address this issue in two ways. First, by preventing the migration of the food oils into the polypropylene layer(s) of the film. Second, by stiffening the film such that the distortions are not so visible to the human eye.
Typically, for preventing migration of the food oils to the polypropylene layer(s) of the film, a metallized layer such as an aluminum layer is applied to the inside the film in contact with the food products. The oil does not penetrate though an intact aluminum layer. However, the problems of applying the metallized layer on the inside of the film are the following. First, the metallized layer tends for form cracks through which the oils leak into the polypropylene layer(s) of the film. Second, it is preferred to have the metallized layer on the side of the film that will be the outside surface of the food package. This is because the metallized layer provides a glossy background to the images printed on the surface of the food package.
The stiffening of the film is typically done by adding inorganic fillers such as calcium carbonate, titanium dioxide, etc., to the core layer of the film. The addition of fillers not only provide increased stiffness but could also provide opacity, e.g., by creating voids using CaCO3, and whiteness, e.g., by using TiO2. See, U.S. Pat. Nos. 4,303,708 (Gebhardt), 4,377,616 (Ashcraft), 4,632,869 (Park), 4,652,489 (Crass), 5,134,173 (Joesten), and 6,048,608 (Peet). The common features of these patents is that they use fillers in a core layer for creating opacity in the film and have a void-free (non-porous) layer on the surface of the film that would contact the food products.
In short, the approach taken by the prior art references was to incorporate a solid (filler-free), void-free layer at the film surface that would be in contact with the food products. The idea behind the prior art films was the following. If one can prevent migration of the oils from the food products into the film using a solid, void-free layer on the film surface that would be in contact with the food products, then one should be able to minimize migration of oils into the film. Thereby, one can prevent distortion of the film. If distortion is still visible, the prior art films attempted to minimize distortion by stiffening the films by adding fillers to the core layer.
The approach utilized by the prior art films discussed above has the following impacts. It generally does not prevent migration of the oils into the core layer, but the distortion is suppressed by stiffening the film and increasing the thickness of the film. Stiffening the film makes the film heavier and less flexible while increasing the film thickness increases the cost of the film. Therefore, it is the objective of this invention to provide an economical solution to this package distortion problem caused by the oils of the food product being packaged.